Abstract: This study investigated the physiological and molecular responses of Malus domestica Borkh. cv.‘Fuji’ to foliar magnesium supplementation using two treatment strategies: application of 1% MgSO₄ at varying spray frequencies and application of a foliar-formulated compound magnesium fertilizer. Four principal indices were evaluated: photosynthetic characteristics of leaves, external fruit quality, internal fruit composition, and expression of sugar metabolism-related genes. Results showed that both treatments significantly increased leaf chlorophyll content, enhanced photosynthetic parameters, improved fruit coloring, reduced titratable acid content, increased soluble solids content, and improved the fruit-to-acid ratio. Additionally, the relative expression levels of genes related to sugar content, such as MdTST1, MdSUT2, and MdERDL6-1, were elevated in both groups, with one treatment in each group yielding the most pronounced effect. These findings demonstrate foliar magnesium interventions not only stimulate photosynthetic activity and nutrient assimilation but also accelerate sugar accumulation and quality formation in apple fruit, thereby offering a viable agronomic approach for optimizing yield and market value in ‘Fuji’ apple cultivation.